The present invention relates to satellite communications systems. In particular, the present invention relates to satellite spot beam pointing and ground station measurements that determine satellite and antenna pointing error.
Satellites have long been used to provide communications capabilities on a global scale. Typically, a modern communications satellite includes multiple downlink antennas, each of which provides communications bandwidth to a large coverage area (or "footprint") using multiple downlink spot beams. Modern satellite antennas operate at much higher carrier frequencies than those in previous systems. Thus, for example, modern satellites may use Ka-band frequency uplinks (at approximately 30 GHz) and downlinks (at approximately 20 GHz), while previous satellites used Ku- or C-Band frequency uplinks and downlinks (approximately 3-12 GHz).
Ka-band frequency downlinks generate relatively small spot beams on the surface of the Earth, and the area each covers is commonly referred to as a "ground cell", or simply a "cell". Transmitted by a geosynchronous satellite, the diameter of Ka-band cell may be only 300-400 miles in diameter (as measured between points in the spot beam where the antenna gain is minimally acceptable, e.g., -5 or -6 dB, relative to the peak antenna gain at the center of the spot beam). Thus, a large number of spot beams may be required to cover a large land mass.
For administration purposes, the satellite and satellite antennas are pointed such that each of the spot beams, preferably, remains fixed over the same portion of the Earth. Thus, for example, a satellite which transits local programming (such as local television) is expected to transmit the local programming to the same cell appropriate for the programming. In order for the local programming to remain local, it is not acceptable for the spot beam to move into a different cell. Maintaining the correct satellite pointing, however, is a difficult task.
Three parameters (attitude components) are generally considered to determine the pointing of a satellite: roll, pitch, and yaw. Typically, the roll axis is defined to be in the direction of the satellite velocity vector and is in the plane of the orbit. The pitch is defined as an axis perpendicular to the roll axis and the orbit plane. Finally, the yaw axis is perpendicular to each of the roll and pitch axis and is in the plane of the orbit. Rotations about the yaw axis may therefore rotate the downlink spot beams about the yaw axis, while rotations about the roll and pitch axes produce movements in the downlink spot beams perpendicular to the velocity vector or parallel to the velocity vector, respectively.
The gravitational influence of the moon, heating and cooling effects (particularly as the satellite is alternately exposed and hidden from the sun), and imperfections in the satellite attitude control system are only three of the many causes of error in the desired satellite pointing. Even slight errors in pointing create enormous differences in spot beam strength between the desired pointing and the erroneous pointing. As an example, a user located only 0.4 degrees from the antenna beam boresight experiences a 4 dB (more than half power) loss of spot beam strength when the satellite pointing is in error by as little as 0.1 degree.
In the past, several attempts have been made to detect and correct satellite pointing error. In U.S. Pat. No. 4,630,058, entitled "Satellite communication system" to Brown, ground stations participate in the detection of pointing error. The ground stations in Brown measure the strength of a beacon signal as transmitted by the satellite's regular communication antenna and as transmitted by a special beacon antenna. The beacon antenna is designed to provide a broader, overlapping, radiation pattern compared to the regular communication antenna.
The ground stations are located near the fringes of the coverage area provided by the regular communication antenna (where the strength of the coverage area falls off rapidly) and are therefore sensitive to pointing error. The pointing error typically causes significant changes in the measured beacon signal as transmitted by the regular communication antenna, while the beacon signal produced by the broader coverage area beacon antenna does not vary nearly as much. By determining the ratio of the measured beacon signals, the degree of pointing error may be determined in two dimensions (generally corresponding to roll and pitch attitude components) assuming that the coverage area gain pattern is known. Appropriate correction signal may then be relayed to the satellite.
Brown, however, is a very complex system. The satellite, in addition to its normal payload, requires additional antenna equipment, switching circuitry and control, and power capacity. Given the expense of launching a satellite, and the need to pack as much revenue producing dedicated communications circuitry onboard as possible, the Brown system tends to be impractical to implement (and further provides no indication of the yaw angle error).
A second attempt at correcting antenna pointing error is disclosed in U.S. Pat. No. 4,910,524, entitled "Investigating and Controlling the Pointing Direction of an Antenna on Board a Spacecraft", to Young et al. In Young, ground stations, located at the fringes of the coverage area produced by the satellite, measure the strength of the RF transmissions in the coverage area. Oscillations in the satellite pointing are deliberately introduced to vary the strength of the RF transmission at the fringes of the coverage area in a predictable manner. Correlating the changes in measured oscillation with the known coverage area gain pattern allows determination of pointing error.
Young, however, requires the satellite to provide a mechanism for causing the spacecraft or the antenna to undergo oscillatory movement. Tremendous complexity is thereby added to the satellite. Generally, the cost of the satellite is correspondingly increased and its reliability decreased.
A third example of an attempt to correct pointing error is shown in U.S. Pat. No. 5,697,050, entitled "Satellite Beam Steering Reference Using Terrestrial Beam Steering Terminals" to Wiedeman. Wiedeman requires at least one reference transmitter on the ground which sends a signal to a reception antenna on board the satellite. The satellite, in bent pipe fashion, relays the signal to a ground station. The ground station determines the gain in the signal and compares the gain to a predetermined gain pattern of the reception antenna. The difference in the actual signal gain and the gain predicted by the gain pattern is used to derive the attitude error of the satellite.
Thus, Wiedeman does not directly correct for pointing errors in the downlink antenna coverage area. Rather, the pointing error corrected for is related to the uplink antenna gain pattern. Additionally, Wiedeman requires reference transmitters (of which there may be several) to consume precious uplink bandwidth by transmitting signals to the satellite. Similarly, the satellite is required to consume precious downlink bandwidth by relaying the signal to a ground station.
A need has long existed in the industry for a method for detecting and correcting satellite spot beam pointing, without requiring elaborate on-board satellite hardware or complex ground measurement equipment.